A typical gas turbine engine includes a compressor section, a combustor and a turbine section. Working fluid flowing through the gas turbine engine is compressed in the compressor section to add energy to the working fluid. Most of the compressed working fluid exits the compressor section and enters the combustor. In the combustor, the working fluid is mixed with a supply of fuel and ignited. The products of combustion are then flowed through the turbine section where energy is extracted from the working fluid. A portion of the extracted energy is transferred back to the compressor section to compress incoming working fluid and the remainder may be used for other functions.
Gas turbine engines are required to function efficiently over a range of operating conditions. For a gas turbine engine used in aircraft applications, low power operation corresponds to idle, high power operation corresponds to take-off and climb, with cruise and approach/descent falling in an intermediate thrust region between low and high power. At low power, fuel/air ratios must be kept relatively rich to avoid blow-out. Blow-out occurs when the fuel/air ratio within the combustor drops below a lean stability limit. As a result of the low combustion temperature and pressure, combustion efficiency is relatively low. At high power, the fuel/air ratio is near stoichiometric to maximize efficiency.
The combustion process generates numerous byproducts such as smoke particulate, unburned hydrocarbons, carbon monoxide, and oxides of nitrogen. At low power, the lower combustion efficiency results in the production of unburned hydrocarbons and carbon monoxide. At high power, the production of oxides of nitrogen increases as the operating temperature and residence time increase. Residence time is defined as the amount of time the combustion mixture remain above a particular temperature. Reducing the operating temperature may reduce the output of the gas turbine engine. Reducing the residence time, may result in less efficient combustion and higher production of carbon monoxide.
For environmental reasons, these byproducts are undesirable. In recent years, much of the research and development related to gas turbine engine combustion has focused on reducing the emission of such byproducts.
A significant development in gas turbine engine combustors has been the introduction of multiple stage combustors. A multiple stage combustor typically includes a pilot stage, a main stage, and possibly one or more intermediate stages. An example of such a combustor is disclosed in U.S. Pat. No. 4,265,615, issued to Lohmann et al and entitled "Fuel Injection System for Low Emission Burners".
At low power only the pilot stage is operated. This permits fuel/air ratios nearer to stoichiometric and the efficiency at idle is thereby increased and the production at idle of unburned hydrocarbons and carbon monoxide is reduced. At high power the pilot stage and one or more of the other stages is operated. Having multiple stages reduces the residence time within each particular stage, relative to a single large combustion chamber. The lower residence time results in lower production of oxides of nitrogen. Having multiple stages also permits the equivalence ratio to be optimized over a range of operating conditions. As a result of having multiple stages rather than a single stage, the emission of unwanted combustion byproducts is reduced and the overall efficiency is improved.
A fuel system for a multiple stage combustor has to be responsive to the operator's demands and provide safe operation throughout the operating range of the gas turbine engine. This is especially true for aircraft applications of gas turbine engines. The gas turbine engine must have high thrust available at all times it may be needed and the switch from low to high thrust needs to be performed quickly and smoothly. For instance, during approach and descent the gas turbine engine is typically operating at an intermediate thrust level, but the combustor must provide the operator with the availability of high thrust upon demand.
In addition to the above considerations, the fuel system should be cost effective and maintainable. Although a highly complex fuel system may provide responsiveness, it may be also be cost prohibitive and increase the downtime of the engine due to increased maintenance. Increased cost and downtime would negate some of the benefits of the multiple stage combustor.
The above art notwithstanding, scientists and engineers under the direction of Applicants' Assignee are working to develop simple, safe and responsive fuels systems for multi-stage combustors.